Geochemical and 3He/4He evidence for mantle and crustal contributions to geothermal fluids in the western Canadian continental margin

Isotopic and geochemical evidence together with helium isotopes are used to identify contributions of deep crustal to upper mantle volatile components in thermal waters at three sites across the western North American plate margin: (1) the low heatflow forearc of the Cascadia subduction zone; (2) the high heat-flow volcanic are; and (3) the interior crystalline terrain of the ancestral continental margin. Western continental margin hotsprings issue 50 degreesC, low salinity Na-Cl water and N-2 gas with 0.25% CH4. Stable isotopes and C-14 indicate local meteoric recharge during the early Holocene. Redox is buffered by sulphate reduction, suggesting that the methane originates from a deeper source. The waters have high helium contents (He/He-air = 30) and a He-3-excess (R/R-air = 0.27; He-3/He-3(air) = 8), representing a mixture of radiogenic He-4 production in crystalline rock with > 10% He from subducted oceanic crust. Geophysical data indicate fluid-filled discontinuities in the subduction zone that may provide a pathway for He, and possibly for CH4 and a saline fluid component from depth. In the Garibaldi belt of Quaternary arc-volcanism, 60 degreesC Na-Cl hotsprings and 200 degreesC geothermal well waters discharge from fractures in the basement rocks. delta O-18 and delta H-2 show the thermal waters to be a mixture of local recharge with up to 8% "andesitic" water from the upper mantle. He isotopes indicate a mantle origin (R/R-air = 6.0), with a minor crustal signature, consistent with observations in the Cascadia range to the south and at other circum-Pacific volcanic are settings. High P-CO2, an enriched delta C-13(DIC) elevated He-3/CO2 ratios and elevated Cl- are likely to be derived from subducted Juan de Fuca prate sediments and pore waters. Thermal Na-SO4 waters (up to 58 degreesC) from the Omineca Crystalline Belt are locally recharged and have an unusually rapid circulation time of less than 40 years. This contrasts with their high radiogenic He content (176 x 10(-7) cc/g) with minimal mantle input (R/R-air = 0.06). The underlying Columbia River Fault Zone is the likely pathway for helium migration from depth. (C) 2000 Elsevier Science B.V. All rights reserved.